1. Field of the Invention
This invention relates to analog filters and more particularly to apparatus for calculating the fast Fourier transform of an analog input signal utilizing charge coupled devices.
2. Description of the Prior Art
In the prior art, the discrete Fourier transform of an analog input signal was performed by sampling the analog input signal with a sample and hold circuit, converting the analog voltage samples into digital numbers with an analog-to-digital converter and with a predetermined number of samples performing numerous additions, subtractions and multiplications by a digital processor to derive digital numbers indicative of the Fourier coefficients of the analog input signal. By placing a constraint on the sample size, such that it is a power of two, the number of additions, subtractions and multiplications may be drastically reduced by using the fast Fourier transform to perform the discrete Fourier transform. The primary disadvantage of the discrete Fourier transform and even the fast Fourier transform is that they require A to D converters, digital processing equipment to perform the numerous calculations and digital storage to hold intermediate data until the final result is obtained. Nevertheless, the desirability of performing the fast Fourier transform from a signal processing standpoint is so great that hardware to perform the fast Fourier transform has been built for many applications using both special and general purpose digital processors.
The fast Fourier transform can be performed using analog adders, subtractors and multipliers in place of digital adders, subtractors and multipliers but heretofore have not because of the precision circuits required and the difficulty in storing analog voltages. These difficulties may however be circumvented by using charge transfer devices (CTD's) or change coupled devices (CCD's) where analog storage of analog signals is easily incorporated.
It is therefore desirable to perform the discrete Fourier transform or fast Fourier transform utilizing discrete analog signals to eliminate analog-to-digital converters and digital signal processing equipment. Furthermore, it is desirable to build similarly structured arithmetic stages which may be incorporated in monolithic or integrated circuit form for low cost and to serve as building blocks for larger systems.